The present invention relates to II-VI semiconductor devices such as laser diodes and light emitting diodes. More specifically, the present invention relates to a passivation capping layer for an ohmic contact to a II-VI semiconductor device.
Group II-VI compound semiconductor devices are known. Such devices are useful in constructing light emitting or detecting devices, diodes and laser diodes such as those described in U.S. Pat. No. 5,213,998, issued May 25, 1993; U.S. Pat. No. 5,248,631, issued Sep. 28, 1993; U.S. Pat. No. 5,274,269, issued Dec. 28, 1993; U.S. Pat. No. 5,291,507, issued Mar. 1, 1994; U.S. Pat. No. 5,319,219, issued Jun. 7, 1994; U.S. Pat. No. 5,395,791, issued Mar. 7, 1995; U.S. Pat. No. 5,396,103, issued Mar. 7, 1995; U.S. Pat. No. 5,404,027, issued Apr. 4, 1995; U.S. Pat. No. 5,363,395, issued Nov. 8, 1994; U.S. Pat. No. 5,515,393, issued May 7, 1996; U.S. Pat. No. 5,420,446, issued May 30, 1995; U.S. Pat. No. 5,423,943, issued Jun. 13, 1995; U.S. Pat. No. 5,538,918, issued Jul. 23, 1996; and U.S. Pat. No. 5,513,199, issued Apr. 30, 1996.
Historically, diodes have generated red or infrared light. However, there are many applications where diodes that emit radiation in shorter wavelengths, for example, in the blue and green portions of the spectrum (i.e., at wavelengths between 590 nm and 430 nm) would be useful. Further, such short wavelength laser diodes would increase the performance and capabilities of many existing systems that currently use infrared and red laser diodes.
It is critical to obtain a good electrical contact to the device. Typically, II-VI semiconductor diodes have employed p-type ZnTe ohmic contacts to provide an electrical conductance to p-type layers of a II-VI semiconductor diode. However, the interface between the ohmic contact and the p-type layer is such that the valence band offset is approximately 1 eV. This offset forms a barrier to hole injection. A graded composition alloy layer has been used as a technique to remove this barrier and is described in U.S. Pat. No. 5,396,103. However, there is a large lattice mismatch between the ohmic contact and the other layers of the device which gives rise to an increased density of microstructural defects in the contact region. These defects may play a significant role in degradation of the performance of the device.
The present invention includes a semiconductor device having a stack of semiconductor layers. A II-VI semiconductor ohmic contact electrically couples to the device and includes beryllium (Be). The ohmic contact has an oxidation rate when exposed to oxidizing environments. A passivation capping layer overlies the ohmic contact and has an oxidation rate which is less than the oxidation rate of the ohmic contact.